1. Toilet: Using the toilet as many times as you want is free, and so is the toilet paper. So drink more water, it not only moisturizes your throat but also allows you to use the free toilet (though it's better if it's in a private room), so use it more often.
2. Waiters: Remember! The service of waiters is also free! So don't hesitate to press the service bell! If they get mad, you can report them, and they will be the ones losing money, so press the bell more often.
3. Magazines: Some private rooms have one magazine, others have two. But buying a magazine costs at least hundreds of dollars, so take it with you.
4. Batteries in the remote control: Basically, you shouldn't leave even the batteries behind, so take them with you.